So, both windows and linux have a pretty useful feature that allows you to leave everything running on the PC while also keeping invaders away by locking the screen. My question is:
Say I leave my laptop with the screen locked while I go get a donnut, and then it gets stolen. Assuming the thief has access to whatever software he needs, how easy/hard would it be for him to access my (currently logged-in) account?

Now let me be clear. I'm not asking if he can access the data on the harddrive. I know he can, and that issue would go under data encryption, which is not my question here. I'm focusing on how hard would it be to get around the "Insert Password" screen, and have full access to my account.

I'm looking for answers regarding both OS's; but, if needed, assume Ubuntu.

Thank you.